The present invention is generally directed to latex processes, and more specifically, to aggregation and coalescence or fusion of the latex generated, and which latex is comprised of a core and a shell thereover, with colorant, like pigment, dye, or mixtures thereof, and optional additive particles. In embodiments, the present invention is directed to toner processes which provide toner compositions with, for example, a volume average diameter of from about 1 micron to about 20 microns, and preferably from about 2 microns to about 10 microns, and a narrow particle size distribution of, for example, from about 1.10 to about 1.35 as measured by the Coulter Counter method, without the need to resort to conventional toner pulverization and classification methods. The resulting toners can be selected for known electrophotographic imaging and printing processes, including digital color processes, and more specifically for imaging processes, especially xerographic processes, which usually require high toner transfer efficiency, such as those with a compact machine design without a cleaning component, or those that are designed to provide high quality colored images with excellent image resolution, acceptable signal-to-noise ratio, and image uniformity, and for imaging systems wherein excellent glossy images are generated.
Aspects of the present invention relate to the preparation and design of a latex polymer with a core-shell structure, or core encapsulated within a shell polymer, and which structure possesses excellent fix and excellent gloss characteristics and wherein the structure can be generated by for example, semicontinuous methods, emulsion polymerization, consecutive emulsion polymerization sequences and the like. The latexes of core and shell which can be prepared by a single stage reaction are preferably of a unimodal molecular weight distribution and single glass transition temperature. A wide variety of latex polymers of for example, differing homopolymeric and copolymeric composition, such as styrene-butadiene-acrylic acid copolymers, styrene-butyl acrylate-acrylic acid copolymers, acrylic homopolymers and copolymers which possess specific chemical, mechanical and/or triboelectrical properties for toner applications can be generated.
There are a number advantages associated with the present invention, for example, in that by using core-shell latexes one can select the optimum properties of each of the core and shell resins, or polymes, such as gloss and fix, which otherwise may not readily obtainable by a single latex. Another advantage of the present invention is that the gloss and fix levels can be varied, (within the limits of individual polymer properties) by adjusting the glass transition temperature, molecular weight, or proportions of each polymer of the core and of the shell. The same principle is also applicable in obtaining glossy or matte finishes. For example, if resin A has a low molecular weight of about 5,000 to about 25,000 there could result for the developed image, an image gloss of greater than 50 gloss units, however the fix may be poor, wherein the MFT is higher than 190.degree. C., or from about 195 to about 225 degrees Centigrade, while if resin B has a high molecular weight of about 40,000 to about 80,000, there could result a poor gloss of for example, an image gloss lower than about 50 gloss units, or from about 30 to about 45 gloss units, and fix wherein the MFT is lower than about 180.degree. C., or from about 150.degree. C. to about 175.degree. C. By combining the above resins them into a core-shell latex, there can be obtained excellent fix and acceptable gloss.
In pictorial or process color applications, the properties of the toner resin such as gloss and fix are important to the attainment of high image quality. Unfortunately, a latex which has the desired fix properties may not yield acceptable gloss properties. For example, if a latex resin has a low molecular weight, that is for example, a Mw of about 5,000 to about 30,000, or lower, it would result in a developed toner image with an excellent gloss, of for example greater than 50 gloss units, such as 70 for high quality color applications (the gloss of the fused images was measured throughout according to TAPPI Standard T480 at a 75.degree. C. angle of incidence and reflection using a Novo-Gloss Statistical Gloss Meter, Model GL-NG 1002S from Paul N. Gardner Company, Inc.), but poor fix, that is the MFT (minimum fixing temperature) is higher than about 190.degree. C. to about 220.degree. C. for the resulting toner. The degree of permanence of the fused images was evaluated throughout by the Crease Test (crease test data can be expressed as MFT), wherein the fused image is folded under a specific weight with the toner image to the inside of the fold. The image is then unfolded and any loose toner wiped from the resulting Crease with a cotton swab. The average width of the paper substrate, which shows through the fused toner image in the vicinity of the Crease, is measured with a custom built image analysis system. The fusing performance of a toner is judged from the fusing temperatures required to achieve acceptable image gloss and fix. For high quality color applications, an image gloss greater than 50 gloss units is preferred. The minimum fuser temperature required to produce a Crease value less than the maximum acceptable Crease is known as the Minimum Fix Temperature (MFT) for a given toner, In general, it is desirable to have an MFT as low as possible, such as for example MFT below 190.degree. C., and preferably below 170.degree. C. in order to minimize the power requirements of the hot roll fuser.) fix; if a latex has a high molecular weight, a Mw of about 35,000 to about 80,000, as determined by Gel Permeation Chromatrography (GPC), then it could result in a poor gloss and excellent fix.
One solution may be to blend various latexes especially designed for toner fix properties and for toner gloss properties, reference for example, U.S. Pat. No. 5,496,676. However, this would involve the addition of at least two latexes to an aqueous solution, and these processes possess inherent problems of limited compatibility between the two different latex resins when the two latex resins are incompatible, such as difference in the individual classes and/or species of the monomeric materials, or in particle surface properties, glass transition temperature, and molecular weight, and this in turn cause the resins to phase separate when heated together into domains rich in each resin, and form separately aggregated particles.
Another solution to preparing a latex having both acceptable gloss and fix is to copolymerize various monomers together; however, this is not always satisfactory primarily because toner gloss and fix are predominantly affected by the molecular weight of the latex in contrasting ways, that is when not using a core-shell polymer latex concept there does not result it is believed a latex polymer with bimodal or multiple modal molecular weight distribution, or a polymer latex with multiple Tg's, for example, a Tg of about 20.degree. C. to about 50.degree. C. in the polymeric core, and a Tg of about 51.degree. C. to about 70.degree. C. in the polymeric shell, as measured by Differential Scanning Calorimetry (DSC), and which can fulfill the requirements for both toner fix and gloss. Thus, the mere copolymerization of various monomers would not it is believed allow the adjustment of the molecular weights which is suitable for both toner fix and gloss applications.